Manually operable instruments of the above-described type are well known and allow joining of elements one to another by so-called heat sealing. As mentioned above, the heating sealing or binding material is inserted into a channel as a solid rod, and it is heated by the electrical heating apparatus to a temperature which is at least equal to the melting temperature of the binding material (e.g. 200.degree. C. to 250.degree. C.). The front part of the rod is inserted into the channel first and is liquefied first. As the rod is introduced further into the channel, the liquefied binding material in the outlet end of the channel is ejected and is used to join two elements to one another.
In the known apparatus, the heating is carried out by means of a resistor and an associated thermostat. When a predetermined maximum temperature is exceeded, the heating resistor is disconnected from its source of electrical energy. The resistor effecting the heating can be mounted either outside of the heatable body or in a recess within the body and is so arranged that the heat is distributed evenly along the channel, so that all places along the channel are heated to a predetermined operating temperature. The thermostat which may, for example, be a bimetallic switch, is directly connected to the heating resistor and opens and closes the supply circuit so as to keep the channel's temperature within a predetermined range.
This apparatus requires a relatively long time for preheating, so that the predetermined operating temperature will be reached all along the channel, allowing an inserted rod to be liquefied in the shortest possible time. This preheating requires use of electrical energy prior to actual use of the equipment. Further, it has been shown that insertion of the rod into the heated channel causes a sufficiently large temperature drop that, even with preheating, an additional delay occurs prior to the time the liquefied binding material appears at the outlet of the channel. This also causes an uneven liquefaction of the binding material, that is, the binding material leaving the channel at the start of the operating cycle may be more viscous than the subsequently available material. This causes a variation in the quality of the joints.